95 C1500 4.3 automatic, about 100k miles hesitating in first & second gear.

About the last week of May I posted a much longer version of this listing a ton of questions. Got 118 views and one pm. Maybe I was in the wrong section.

Because it is OBDI, and my scanner won't work, I short the A and B jumpers to read codes. I don't really get a lot of codes but the ones I was getting were primarily in the emissions area. So I replaced the EGR valve, the EGR solenoid, the MAP sensor, the IAC, the Oxy sensor, fuel filter, and all other filters.

The problem WAS that when it got hot it would hesitate and stumble. I put in a 160 degree thermostat, a temp switch, an engine coolant sensor that feeds the ECU, two new knock sensors, and a new water pump. I also put in a new radiator because the old one had 3 bad holes in it.That hardly helped at all. By the way, at one time or another the OBDI said I had problems with all of the above emissions sensors. I also changed every vacuum hose on the top of the engine relative to emissions.

The problem persisted and finally I began getting codes that suggested the wiring to the ECU was bad, or I needed to replace the ECU.

Everyone at parts shops told me I needed a OEM #16196395. This is a plug and play computer, preprogrammed at the factory but when I checked mine out it called for an OEM #1619742. This computer is supposed to be flashed at the factory and again in the vehicle after it is installed, and has a Powertrain Control Module.

When I talked to the manager at Autozone he called the manager at Cardone while I stood there. The guy said it absolutely did not have to be flashed, just install and go. So I bought it, installed it according to directions, and it fired right up.
Shortly after I began having the hesitating problems and a couple times it went into "limp" mode. It gave me codes saying the distributor wiring was bad or the distributor was.

I checked the wiring and it was fine. I replaced the distributor with a new one, not rebuilt. Also a new cap and rotor. It took me forever to get it timed right. I was always 180 degrees off. Finally, an old timer who worked for GM before he retired told me that engine, for 2 years, has two timing marks that have to be lined up correctly. When one is at zero degrees at TDC, the second one should be about 60 degrees to the right of the marker. You can reach under and feel it. Of course, if you don't have the #1 cylinder at TDC on the compression stroke you will be 180 off, in which case you can turn the crank around until the SECOND timing mark comes up. Put the flat part of the dist cap pointing to the firewall, and the rotor at the #1 cyl position on the distributor cap and you should be there.

I did all this, it started right up, and I was ONE degree away from being zero degrees at TDC. I hooked everything up for a road test. Nine miles and it was perfect, then it started the hesitation again, primarily in low and second gears. Again, I have gone up to 5 degrees retarded to 5 degrees advanced.

I know I've thrown some of these parts at it trying to solve it, but it is a truck I will eventually give to my granddaughter, and I want it as perfect as can be.

Help me guys. Do you think it needs to be flashed now? Rebuild the TBI with new gaskets and O rings? (It sprays perfect 360 degree cones out of the injectors.) Any suggestions will be welcome.

Just a thought here, but have you checked the cat? Mine hesitates and loses power once hot. I checked the cat with a thermal imaging camera and it is showing 2100 degrees at the front of the cat and 1400 at the back. (You can also do this test with a cheap IR temp gun) As the cat heats up, the materials inside begin to expand and restrict flow. Generally, a spread of 400 degrees from front to back indicates a plugged cat. I am waiting for some assistance to drop mine down, fix and replace the seals.

Just a thought here, but have you checked the cat? Mine hesitates and loses power once hot. I checked the cat with a thermal imaging camera and it is showing 2100 degrees at the front of the cat and 1400 at the back. (You can also do this test with a cheap IR temp gun) As the cat heats up, the materials inside begin to expand and restrict flow. Generally, a spread of 400 degrees from front to back indicates a plugged cat. I am waiting for some assistance to drop mine down, fix and replace the seals.

Click to expand...

You know, I never even thought of the cat. I'll have to get my hands on an IR gun. Would you happen to know what a normal front and rear temp should be? If it is the cat, that would explain why so many of the things I did didn't seem to make a difference. I appreciate you posting that. I'll check that out and let you know if that's it!

You know, I never even thought of the cat. I'll have to get my hands on an IR gun. Would you happen to know what a normal front and rear temp should be? If it is the cat, that would explain why so many of the things I did didn't seem to make a difference. I appreciate you posting that. I'll check that out and let you know if that's it!

Click to expand...

Normal temps vary from vehicle to vehicle, but I wouldn't think you want to see anything over 1400. Look for a wide temp variation between the inlet and the outlet of the cat. 400 degrees or more is an indication that there is a problem. Similarly, you can look at the cat at night in the dark after driving. A plugged cat will oftentimes present by glowing red hot. not usually so much so that you could see it during the day, though.

Also, When was the last time you did plugs and wires? What brand were they? If they are Bosch plugs, take them out and put AC Delco plugs in. Same goes for your wires. If they are not AC Delco, replace 'em. Sounds like a long shot, but I have seen many times first hand when a vehicle was running poorly, it turned out to be Bosch plugs or auto zone plug wires.

Normal temps vary from vehicle to vehicle, but I wouldn't think you want to see anything over 1400. Look for a wide temp variation between the inlet and the outlet of the cat. 400 degrees or more is an indication that there is a problem. Similarly, you can look at the cat at night in the dark after driving. A plugged cat will oftentimes present by glowing red hot. not usually so much so that you could see it during the day, though.

Also, When was the last time you did plugs and wires? What brand were they? If they are Bosch plugs, take them out and put AC Delco plugs in. Same goes for your wires. If they are not AC Delco, replace 'em. Sounds like a long shot, but I have seen many times first hand when a vehicle was running poorly, it turned out to be Bosch plugs or auto zone plug wires.

Click to expand...

I'm a firm believer in AC Delco too, and that's what my plugs and wires are. Probably don't have 35 miles on them. I also found a test procedure from Autozone's procedures where you take out the oxy sensor (it is before the cat) and hook up a vacuum gauge. I don't remember off the top of my head the exact numbers, but it should idle at about 1.25 psi to 4.0 psi. You then rev it up to 2500 rpms for something like 2 minutes and if it goes over 8 psi you know the cat is bad. They have a couple more tests too, but I got a buddy who has a temp gun he'll bring over Sat and we'll check it out.

Now, for the head scratcher. I was just going back over all my codes and stuff and one kept referring me to ignition (in the Haynes manual.) It says there that even if you time the engine dead on, once you take the battery cable off the ECC loses everything it knew. You have to start it, let it idle til warm, then hold it at 2000 rpms for 2 minutes. Then you have to drive it a minimum of 5 miles so the ECU can go through the IAC, the timing, the MAP, basically everything that feeds input to the ECU. And then, the sky will turn rosy and you'll drive off happily into the sunset. You ever hear anything like that? (Not the sky part really). It says that during all this time it will buck and hesitate and generally turn out a poor performance until it reprograms itself. Then, as each part is programmed back into memory, it will start to run better until finally it is where you want it, assuming you don't have bad timing or a bad cat or something.

Your truck's computer has what is called adaptive memory. As the truck and engine components age and tolerances and values change, the computer makes adjustments to keep things running right. When you disconnect the computer, the adaptive memory is cleared and lost. There is a procedure to have the computer relearn all of the little nuances it picked up over the years. For example, when your throttle plates get dirty over time and no longer close all the way, the adaptive memory makes adjustments to the iac valve and other variables to compensate, and so on.

The best way to have the computer rebuild the variables it will need to run properly is to just drive it normally for a few drive cycles, rather than to stand on one leg while holding the passengers side window down button while keeping the engine at 2000 rpm for 37 seconds, or whatever the procedure is.

Something else to check out would be the TPS switch if you haven't already done so. The throttle position sensor will cause the engine to surge and buck. Generally not only at operating temp, but heat does make things break down, so you never can tell.

I will continue to rack my brain for other potential causes too, but in the meantime, this should give you stuff to check. Let us know what you find.

Your truck's computer has what is called adaptive memory. As the truck and engine components age and tolerances and values change, the computer makes adjustments to keep things running right. When you disconnect the computer, the adaptive memory is cleared and lost. There is a procedure to have the computer relearn all of the little nuances it picked up over the years. For example, when your throttle plates get dirty over time and no longer close all the way, the adaptive memory makes adjustments to the iac valve and other variables to compensate, and so on.

The best way to have the computer rebuild the variables it will need to run properly is to just drive it normally for a few drive cycles, rather than to stand on one leg while holding the passengers side window down button while keeping the engine at 2000 rpm for 37 seconds, or whatever the procedure is.

Something else to check out would be the TPS switch if you haven't already done so. The throttle position sensor will cause the engine to surge and buck. Generally not only at operating temp, but heat does make things break down, so you never can tell.

I will continue to rack my brain for other potential causes too, but in the meantime, this should give you stuff to check. Let us know what you find.

Click to expand...

Hey Moogvo, my friend brought his temp gun over this am. It's a pretty good one, he bought it from work. I started the truck and let it idle about 15 minutes, and he took the readings, four times to make sure. There was just over a 100 degree difference from front to rear. I was surprised, because it has about 100k on it.
I also used the procedure in the Haynes book to check out the TPS. I had 5 volts with the key on, but although I was in range of the temps with the throttle open to closed, it was not a smooth transition, but a jerky jump up and/or down in voltage. Maybe time to change that, huh? Changed about everything else.
Lastly, I set the timing again on zero @ TDC and took a 15 mile trip. From the time I left the driveway it jerked and bucked, and I could only get it to change gear by backing almost all the way off the throttle and barely feathering it up. Never got over 40 mph. In fact, twice it went into limp mode, and I turned on the flashers so people could pass me. I stepped on the passing gear and it backfired for the first time since I've been doing all this. I hate to say it, but it seems to be much worse. I torqued all the nuts on the intake down, thinking maybe I had a leak that I couldn't hear. I just changed the fuel filter last month. I took the hose off the EGR valve while the truck was idling and it made no difference. Plugged up the tube and the spout it goes on and it made no difference, revving up or idling. Took the EGR valve off and it was black with carbon, you'd have thought it was ten years old. I cleaned it out with SeaFoam (fixes everything else, I thought why not?) shot a little WD40 into it, and hooked it back up. It started raining so I pulled it inside the garage. Oh, the grounds for the ECM/PCM were a little ragged with several strands of wire sticking out. I cut them off and crimped on new connectors, wire brushed the bolt they go on and fastened them back up. I am stumped.

It wouldn't run if the timing was 180 off. Try the throttle position sensor.

Click to expand...

Hey Tom, I replaced the TPS and guess what purs like a kitten! It did take a few 3 to 5 mile runs, park it, let it set a while, and take it out again. Each time, I could feel a difference in the way it drove. After about 5 mini trips, I guess the computer got it together, because it is the smoothest running vehicle I own, and I have 5! Thanks for all your support, and your ideas, that made a ton of difference!

Hey Tom, I replaced the TPS and guess what purs like a kitten! It did take a few 3 to 5 mile runs, park it, let it set a while, and take it out again. Each time, I could feel a difference in the way it drove. After about 5 mini trips, I guess the computer got it together, because it is the smoothest running vehicle I own, and I have 5! Thanks for all your support, and your ideas, that made a ton of difference!